Every molder knows the feeling: the quote promised a 32-second cycle, the first articles come out at 48, and the customer is asking why. In prototype and low-volume plastic injection molding, cycle time is not one number — it is four stages stacked together, and two of them (cooling and packing) usually account for over half of it. Break the cycle down and the fixes become obvious.
Anatomy of a Cycle
A typical cycle for a 2 mm wall part looks roughly like this:
- Fill: 0.5–2 s, rarely worth optimizing first
- Pack and hold: 5–15 s, set by how long the gate stays sealed
- Cooling: 15–40 s, the single largest block
- Mold open, ejection, close: 2–6 s on a modern machine
Cooling time scales roughly with the square of wall thickness. That single sentence explains more mold-shop arguments than anything else: going from a 2.5 mm wall to 2.0 mm can cut the cooling block nearly in half. If a part is running long cycles, the first question is not "can the machine go faster" but "why is this wall so thick".
Cooling: Where the Real Money Is
The plastic in the cavity only sees mold steel on one side per surface, and heat has to travel through that steel into coolant. Three factors dominate:
- Coolant temperature and flow. Running coolant at 15 °C on ABS sounds aggressive, but molders typically run 40–80 °C to control residual stress and gloss; PC often needs coolant at 80–100 °C to avoid stress cracking. You cannot cheat material recommendations without paying in warp.
- Turbulent flow. Laminar water in a cooling channel moves heat poorly. A rule of thumb used on the shop floor: aim for a Reynolds number above 5,000, which usually means about 10 L/min through a 10 mm channel. Check flow and delta-T between inlet and outlet — a rise of more than about 3 °C across the tool says the circuit is starved.
- Channel placement. Cooling lines should run within roughly 1–2× the channel diameter of the cavity surface, uniformly on both sides. Cores are the usual villain: a deep boss has no water inside it, so that boss dictates the cycle. Baffles, bubblers or high-conductivity (beryllium copper / AMPCO) inserts in cores are the standard fixes.
Hold Time: Shorter Than You Think
Hold pressure compensates shrinkage until the gate freezes. Anything after gate freeze is wasted seconds — the material cannot get in. Practical sizing: hold time a little longer than the measured gate seal time, which you find by molding a few shots at stepped hold times and plotting part weight; the weight plateaus at seal.
Gate size is the lever. A gate that is 0.5 mm deep where 0.4 mm would do can add 2–4 seconds of hold on every shot, forever. This is one reason we write that plastic injection gate design is the highest-leverage decision in the whole tool — see our earlier gate design guide for sizing rules by material.
Material Choice Changes the Math
The same part in different resins is a different cycle:
- ABS: forgiving, molds at moderate temperatures, a solid default for appearance prototypes.
- PC: needs hot tools and higher hold, runs maybe 20–40% longer cycles than ABS in the same geometry.
- PP: cheap and fast, but shrinks 1.2–2.0% — dimensional control on a prototype tool is harder.
- Glass-filled grades (PA6-GF30 and friends): stiffer and lower shrink across flow, but warpage from differential shrink and higher tool wear; also they hide less behind longer hold times.
What You Can Actually Do on a Prototype Program
When you are ordering 50–500 parts, not millions, some textbook optimizations are not worth it. Conformal-cooled inserts rarely pay back on a prototype tool. But these do:
- Uniform wall — the cheapest cycle-time reduction available, and it is free at the CAD stage.
- Early gate seal — spec a smaller gate on the DFM review; on a soft tool you can open it up later if short shots appear, but you cannot close one that is too big.
- More, better-routed cooling circuits instead of one loop threaded wherever it fit.
- Robot or drop ejection instead of hand unloading, worth 2–4 s per shot once volumes justify it.
There is also a strategic option: if your "prototype" volumes are really creeping toward production numbers, a soft tooling bridge program with aluminum tools lets you tune the process on production-representative steel behavior without committing to a full production mold. And when volumes are still tiny, 3D printing sidesteps cycle time entirely — the crossover point depends on part count and tool cost, not on wishful thinking.
A Simple Review Checklist
- Thickest wall section: does it really need to be that thick?
- Gate land length minimized for the resin?
- Water circuits within 1–2× diameter of the cavity, both sides, verified flow?
- Deep cores fitted with baffles or high-conductivity inserts?
- Coolant temperature matched to the material data sheet, not to the chiller's convenience?
Cycle time problems are almost always designed in before the steel is cut. Sending the part model for a DFM pass — wall map, gate plan and cooling layout — catches the expensive ones early. Share your model with us and we will go through the four stages of your cycle one by one.
